When you create a terrain to use as a mountain. Go into the Terrain editor - start at the default grid size of 128 and click a couple of times on the "slope noise" button, and a couple of times on the "erode" button above. Then change the grid size to 256 and repeat (hit the "smooth" button ONCE at this point), then adjust the grid to 512 and repeat the slope noise and erode process again. If your mountains are relatively close (scene-wise), or you want really good detail, change the grid size to the max of 1024 and do it one more time. This is the basic way to get realistic mountains - play around with it and experiment (more noise, less erode, vice versa, etc.), and soon you'll be able to do realistic mountains. When done, don't forget to raise the bottom 'cut off' a little so you don't have a square base to your mountain. The other important thing to mention is do NOT make your mountains small and close to the camera. Make them BIG and far away (Very big and Very far)- they'll look much more real. Check my gallery for examples of what I've been able to get with this technique - especially my recent pics like 'Wu Tien Shan' and 'Lungmen Jia'.